A total conversion. So you need at least a mapping tool, a modelling tool, the game source code and a compiler for C. As mapping tool I recommend GTKRadiant V 1.4.0 (free available). The game source code is also free available. Not sure about the compiler. I think there is a free version from Microsoft available. Alternatively you should try to compile under Linux (Cross compiling for Windows is possible with Linux). If you want to alter the player models and playermodel animations I recommend Lightray3d (shareware). The alternative Milkshape can't deal with the player models and playermodel animations. Another disadvantage of Milkshape is the missing of the capability to work with model tags (needed for the work with weapons).